(i) Field of the Invention
The present invention relates to a process for the decomposition of methanol. More specifically, the present invention relates to a process for decomposing methanol to obtain a gas containing H.sub.2 and CO as main components.
(ii) Description of the Prior Art
In countries where raw materials for methanol are available at low costs, large-scale plants on a level of 3,000 tons/day have been built, and a large amount of methanol is exported from these countries to other consuming countries. Most of methanol is used as a raw material for chemicals such as formaldehyde. Furthermore, much attention is being paid to methanol as an inexpensive and clean energy source and a source of CO and H.sub.2.
The gas containing CO and H.sub.2 as the main components which can be obtained by decomposing methanol can also be used as a fuel for an internal combustion engine and the like. That is, a high-temperature exhaust gas from an internal combustion engine is utilized as a source of reaction heat necessary to decompose methanol, whereby the energy of the exhaust gas can be effectively recovered and thus the thermal economy of the internal combustion engine can be improved. Additionally, in this case, the production of aldehyde is inhibited and a clean exhaust gas is obtained advantageously.
Furthermore, CO and H.sub.2 obtained by the decomposition of methanol are useful as raw materials for chemicals. That is, these compounds are important as essential raw materials for the manufacture of aldehydes by oxo synthesis, the manufacture of acetic acid by the carbonylation of methanol, the manufacture of diamines by the H.sub.2 reduction of dinitrotoluene, the manufacture of phosgene by the reaction of CO with chlorine, the manufacture of an isocyanate (TDI) by the reaction of a diamine with phosgene, and the like.
In general, CO and H.sub.2 are industrially obtained by the steam reforming or the partial oxidation of hydrocarbons such as methane, naphtha, crude oil and coal. On the other hand, the process for forming CO and H.sub.2 by the decomposition of methanol has advantages such as the employment of lower reaction temperatures, a lower capital investment in facilities and less labor for operation, as compared with the above-mentioned conventional methods. It is fair to say that the process for the preparation of CO and H.sub.2 by the decomposition of methanol is economical due to the use of inexpensive methanol.
In the decomposition of methanol, a catalyst is used, and many catalysts for this application have been suggested. They are catalysts, on a carrier such as previously treated alumina, which are active metallic compounds, particularly platinum group elements, base metal elements such as copper, nickel, chromium and zinc, and their oxides. In addition, it has also been suggested to directly use, in the decomposition reaction, a mixed catalyst comprising oxides of the base metals, particularly a catalyst for methanol synthesis having a chromium oxide/zinc oxide system, a copper oxide/zinc oxide system or a copper oxide/zinc oxide/aluminum oxide system without using any carrier.
The conventional catalysts have the following drawbacks:
(1) The catalyst on a carrier is poor in durability. Particularly with regard to the catalyst containing a noble metal or a copper group element on a carrier, its catalytic activity tends to deteriorate due to impurities present in the raw material, and what is worse, the copper group element catalyst is poor in heat resistance. PA1 (2) As compared with the catalyst having a carrier, the catalyst for methanol synthesis is superior in durability, but when the latter catalyst is used, dimethyl ether, methane and high-boiling products are produced as by-products. PA1 (3) It is known that when a conventional catalyst is used, the decomposition reaction of methanol is slow and carbon is deposited on the catalyst. Thus, when the catalyst is used for a long period of time, the catalytic activity tends to deteriorate (AICHE, Spring National Meeting, "Methanol Dissociation for Fuel Use" 1984; EP B1 18700; U.S. Pat. No. 4,780,300; and Japanese Laid-open Patent Publication Nos. 51-119002, 51-122102 and 52-52902). In order to solve the above-mentioned problems, a method has been suggested in which methanol and water are fed to a catalyst layer to partially bring about the steam reforming of methanol, and a carbonaceous material or its precursor deposited on the catalyst is then removed therefrom by steam distillation.
As described above, when methanol is fed together with water to the catalyst layer, the CO/H.sub.2 ratio in the produced gas falls. Such a gas composition is inconveniently unsuitable for the synthesis of aldehydes by oxo synthesis or the synthesis of acetic acid by using methanol and CO in which CO only is required.
In the conventional methanol decomposition process, the catalyst is low in stability and durability as described above, and the deterioration of the catalyst is observed at high temperatures. Furthermore, the gas obtained by the conventional decomposition process contains a considerable amount of by-products such as methane, dimethyl ether and high-boiling products, and therefore this kind of gas is required to be purified, when CO and H.sub.2 in the gas are used as the raw materials for producing chemicals.